A conventional high-pressure metal vapor discharge lamp using a ceramic material for the discharge tube is disclosed, for example, in Publication of Unexamined Japanese Patent Application No. Hei 6-196131.
This conventional high-pressure metal vapor discharge lamp uses a discharge tube where the two ends of a cylindrical portion are plugged with disks by shrinkage fitting. Regardless of the lamp orientation of this high-pressure metal vapor discharge lamp during operation, in other words for vertical operation, where the metal vapor discharge lamp is arranged so that the axes direction of the electrodes point in a vertical direction, as well as for horizontal operation, where the metal vapor discharge lamp is arranged so that the axes of the electrodes point in a horizontal direction, a condensed phase of the excess discharge metal compound is present in the shrinkage-fitted plug portion. Thus, a high-pressure metal vapor discharge lamp whose operating characteristics are independent from the lamp orientation can be obtained.
However, since in this conventional high-pressure metal vapor discharge lamp the ends of two cylindrical portions of the discharge tube are plugged with disks by shrinkage fitting, the airtightness of the plug portion is not very reliable, and the lamp characteristics cannot be maintained sufficiently over long-term use.
Another configuration that has been proposed for high-pressure metal vapor discharge lamps using a ceramic discharge tube relates to a discharge tube with cylindrical portions and tapered portions, wherein the ends of two cylindrical portions are plugged by shrinkage fitting without disks. This high-pressure metal vapor discharge lamp can ensure airtightness with higher reliability, because the discharge tube is shrinkage-fitted without disks. However, its operating characteristics depend on the lamp orientation, and vary when the position of the condensed phase of the excess discharge metal compound changes.
It is an object of the present invention to solve the problems of the prior art. It is a further object of the present invention to provide a metal vapor discharge lamp wherein (a) the discharge tube does not include disks among its parts and (b) the shape of the discharge tube is optimized, so that good operating Characteristics are maintained over long-term use, and the operating characteristics depend only little on the lamp orientation.